The motion picture projectionist (Nov 1931-Jan 1933)

December, 1931 Motion Picture Projectionist 33 The How and Why of Lenses (Continued from page 13) Point F., Figure 3-G, is called the principal focus of the lens and is measured by the radius of the curvature of the lens. Now, suppose we place a source of light L, which hereinafter we shall call a radiant, at the principal focus F. Light rays from this point and incident on the surface of the lens will be refracted and upon emerging they will take parallel paths as shown. This, you will note, is just the reverse of what happened in Figure 3-F. Figure 3-H shows that if a radiant L is placed to the left of the principal focus of a converging lens the rays upon emerging from the lens will be bent downward and will meet at a point LI beyond the principal focus Fl on the opposite side of the lens. If the radiant is placed at LI the rays will converge at point L. This pair of points at which the rays converge on both sides of the lens is called the "conjugate foci'' and are so related that LI is an image of L, or if LI is the radiant then L is the image of LI. If the radiant L in Figure 3-1 is placed between the principal focus F and the lens we will find that the emerging rays will diverge, spread out, and never meet in a focus on that side of the lens. However, if these divergent rays are traced backward, as shown by the dotted lines, they will meet at F. Our concern with lenses, therefore, is to know that with them we are able to collect rays of light and direct them according to our requirements. With the knowledge of how light is affected, or acted upon, in passing through various types of lenses, we are better able to design picture and sound head optical systems, and in the case of the projectionist, better able to adjust and to service the equipment intelligently. Sound Head Optical Systems (Continued from page 17) second) of the recorded sound. Hence to obtain the length of sound track occupied by one cycle (or one complete vibration) of the recorded sound the procedure would be to divide the length of track passing under the beam in one second by the frequency of the recorded sound. Omitting the actual work of division the length of track required for one complete cycle of each of the following frequencies is: 100 cycles 0.180 inch 1000 cycles 0.018 " 10000 cycles 0.0018 " 18000 cycles 0.001 " Remembering that the only condition under which sound is actually reproduced is that obtaining when the light falling upon the photocell varies, it may be seen easily that 'Start Motor! GOLDE Three-Alarm Says: 1. "Get Ready!" 2. "Start Motor!" 3. "Change Over!" You can't help but give a smooth, unbroken show with this marvelous timing device. Gives three distinct signals. You can't miss them. You can't mistake them. Good shows help the box office. Plenty of warning relieves you of worry. You don't have to watch the screen at all: a wink or sneeze can't make you miss a cue. Your machine is at full speed when wanted — no chance for slow pickup to spoil the show. Installed in five minutes. Adjusted in five more. Never needs further attention. Never fails. Nothing to get out of order. Made of steel, bronze, and duralumin finished in black crackle lacquer, outlasts the projection machine. Pays for itself in convenience in one performance. Pays for itself at the box office in a month. Write for illustrated folder with full description and moderate price. GOLDE MANUFACTURING COMPANY 2013 LeMoyne Street, Chicago, 111. I. "Get Ready! for TRUE and LIFELIKE REPRODUCTION use "TELEPHOTO" Photoelectric Cells The Caesium-Argon Cell of QUALITY. End your cell worries by adopting "TELEPHOTO" as your standard equipment WRITE for Pamphlet No. 5 and prices. Teleplioto & Television Corp. 133-135 West 19th Street, New York City with a light beam falling on the sound track having a width of 0.001 inch and with a recording on that sound track of which each cycle (or complete vibration) occupies a space of 0.001 inch there will be no variation of light falling upon the photocell as the film recording passes under the scanning beam and hence no sound. Similarly, it may be proved both by actual experiment and mathematically that the variation of light falling upon the photocell increases as the disparity between the slit width and the length of track occupied by one cycle of the sound increases. The point at which computation shows that no response will be obtained from a given slit and lens assembly is called the theoretical point of cut-off of the combination to distinguish it from the point of practical cut-off of the combination. The practical cut-off of any such assembly is roughly one-half of the theoretical cut-off and represents the point at which the response of the slit and lens assembly begins to dwindle below the point of practical usage. Its Importance to Reproduction Slit width, or, more properly, the width of the light beam at the film, is of the greatest importance in the satisfactory reproduction of sound. Effectively, a slit is widened when it is rotated to any position other tha3i that in which it is horizontal and parallel to the plane of the film. That this is true may be seen from consideration of the effect of rotating the entire lens barrel. If such rotation be done to such a degree that the light beam is in a vertical position the extreme of this effect is observed. The light beam is now vertical and extends approximately 0.084 inch instead of 0.001 inch as before. Rotating the lens barrel from this position 45 degrees towards its normal position also rotates the light beam and the width